NUCLEOLUS: CHARACTERISTICS, STRUCTURE, MORPHOLOGY AND FUNCTIONS - BIOLOGY - 2025

The nucleolus is a cell structure not delimited by a membrane, being one of the most prominent areas of the nucleus. It is observed as a denser region in the nucleus and is subdivided into three regions: dense fibrillar component, fibrillar center, and granular component.

It is mainly responsible for the synthesis and assembly of ribosomes; however, this structure also has other functions. More than 700 proteins have been found within the nucleolus that are not involved in ribosome biogenesis processes. In the same way, the nucleolus is involved in the development of different pathologies.

The first researcher to observe the nucleolus zone was F. Fontana in 1781, more than two centuries ago. Then, in the mid-1930s, McClintock was able to observe such a structure in his experiments with Zea mays. Since then hundreds of investigations have focused on understanding the functions and dynamics of this region of the nucleus.

General characteristics

The nucleolus is a prominent structure located within the nucleus of eukaryotic cells. It is a “region” in the shape of a sphere, since there is no type of biomembrane that separates it from the rest of the nuclear components.

It can be seen under the microscope as a subregion of the nucleus when the cell is at the interface.

It is organized in regions called NORs (for its acronym in English: chromosomal nucleolar organizer regions), where the sequences that encode ribosomes are found.

These genes are in specific regions of the chromosomes. In humans they are organized in tandem in the satellite regions of chromosomes 13, 14, 15, 21 and 22.

In the nucleolus, the transcription, processing and assembly of the subunits that make up the ribosomes occurs.

In addition to its traditional function, the nucleolus is related to tumor suppressor proteins, cell cycle regulators and even proteins from viruses.

Nucleolus proteins are dynamic, and their sequence appears to have been conserved throughout evolution. Of these proteins only 30% have been associated with ribosome biogenesis.

Structure and morphology

The nucleolus is divided into three main components, distinguishable by electron microscopy: the dense fibrillar component, the fibrillar center and the granular component.

Generally, it is surrounded by condensed chromatin, called heterochromatin. In the nucleolus, the processes of transcription of ribosomal RNA, processing and assembly of ribosomal precursors occur.

The nucleolus is a dynamic region, where the proteins that the components can quickly associate and separate from the nucleolar components, creating a continuous exchange with the nucleoplasm (gelatinous substance internal to the nucleus).

In mammals, the structure of the nucleolus varies with the stages of the cell cycle. In prophase a disorganization of the nucleolus is observed and it reassembles at the end of the mitotic process. Maximum transcriptional activity in the nucleolus has been observed in the S and G2 phases.

The activity of RNA polymerase I can be affected by different states of phosphorylation, thus modifying the activity of the nucleolus during the cell cycle. Silencing during mitosis occurs due to the phosphorylation of different elements such as SL1 and TTF-1.

However, this pattern is not common in all organisms. For example, in yeast the nucleolus is present - and active - during the entire process of cell division.

Fibrillary centers

The genes that code for ribosomal RNA are located in the fibrillar centers. These centers are clear regions surrounded by dense fibrillar components. The fibrillar centers are variable in size and number, depending on the cell type.

A certain pattern has been described with respect to the characteristics of the fibrillar centers. Cells with high ribosome synthesis have a low number of fibrillar centers, while cells with reduced metabolisms (such as lymphocytes) have larger fibrillar centers.

There are specific cases, as in neurons with a very active metabolism, whose nucleolus has a giant fibrillar center, accompanied by smaller smaller centers.

Dense fibrillar component and granular component

The dense fibrillar component and the fibrillar centers are embedded in the granular component, the granules of which have a diameter of 15 to 20 nm. The transcription process (passage of the DNA molecule to RNA, considered the first step of gene expression) occurs at the limits of the fibrillar centers and in the dense fibrillar component.

Processing of ribosomal pre-RNA occurs in the dense fibrillar component and the process extends to the granular component. The transcripts accumulate in the dense fibrillar component and nucleolar proteins are also located in the dense fibrillar component. It is in this region where the assembly of ribosomes occurs.

After this assembly process of ribosomal RNA with the necessary proteins is completed, these products are exported to the cytoplasm.

The granular component is rich in transcription factors (SUMO-1 and Ubc9 are some examples). Typically, the nucleolus is surrounded by heterochromatin; This compacted DNA is thought to play a role in ribosomal RNA transcription.

In mammals, ribosomal DNA in cells is compacted or silenced. This organization appears to be important for the regulation of ribosomal DNA and for the protection of genomic stability.

Nucleolar organizing region

In this region (NOR) the genes (ribosomal DNA) that code for ribosomal RNA are grouped.

The chromosomes that make up these regions vary depending on the species under study. In humans they are found in the satellite regions of the acrocentric chromosomes (the centromere is located near one of the ends), specifically in pairs 13, 14, 15, 21 and 22.

The ribosome DNA units consist of the transcribed sequence and an external spacer necessary for transcription by RNA polymerase I.

In promoters for ribosomal DNA, two elements can be distinguished: a central one and an element located upstream (upstream).

Features

Ribosomal RNA Forming Machinery

The nucleolus can be considered a factory with all the components necessary for the biosynthesis of ribosome precursors.

Ribosomal or ribosomal RNA (ribonucleic acid), commonly abbreviated as rRNA, is a component of ribosomes and participates in the synthesis of proteins. This component is vital for all lineages of living beings.

Ribosomal RNA associates with other components of a protein nature. This binding results in ribosomal presubunits. The classification of ribosomal RNA is generally given accompanied by a letter "S", which indicates the Svedberg units or sedimentation coefficient.

Organization of ribosomes

Ribosomes are made up of two subunits: the major or large and the small or minor.

The ribosomal RNA of prokaryotes and eukaryotes is differentiable. In prokaryotes, the large subunit is 50S and is composed of 5S and 23S ribosomal RNAs, likewise the small subunit is 30S and is composed only of 16S ribosomal RNA.

In contrast, the major subunit (60S) is composed of 5S, 5.8S, and 28S ribosomal RNAs. The small subunit (40S) is composed exclusively of 18S ribosomal RNA.

In the nucleolus are the genes that code for ribosomal RNAs 5.8S, 18S and 28S. These ribosomal RNAs are transcribed as a single unit within the nucleolus by RNA polymerase I. This process results in a 45S RNA precursor.

Said ribosomal RNA precursor (45S) must be cleaved into its 18S components, belonging to the small subunit (40S) and to 5.8S and 28S of the large subunit (60S).

The missing ribosomal RNA, 5S, is synthesized outside the nucleolus; Unlike its counterparts, the process is catalyzed by RNA polymerase III.

Ribosomal RNA transcription

A cell needs a high number of ribosomal RNA molecules. There are multiple copies of the genes that code for this type of RNA to meet these high requirements.

For example, based on data found in the human genome, there are 200 copies for 5.8S, 18S, and 28S ribosomal RNAs. For 5S ribosomal RNA there are 2000 copies.

The process begins with 45S ribosomal RNA. It starts with removing the spacer near the 5 ′ end. When the transcription process is complete, the remaining spacer located at the 3 ′ end is removed. After subsequent deletions, the mature ribosomal RNA is obtained.

Furthermore, the processing of ribosomal RNA requires a series of important modifications in its bases, such as processes of methylation and conversion of uridine to pseudouridine.

Subsequently, the addition of proteins and RNAs located in the nucleolus occurs. Among these are the small nucleolar RNAs (pRNA), which participate in the separation of ribosomal RNAs in the 18S, 5.8S and 28S products.

PRNAs possess sequences complementary to 18S and 28S ribosomal RNAs. Therefore, they can modify the bases of the precursor RNA, methylating certain regions and participating in the formation of pseudouridine.

Ribosome assembly

The formation of ribosomes involves the binding of the parent ribosomal RNA, together with ribosomal proteins and 5S. The proteins involved in the process are transcribed by RNA polymerase II in the cytoplasm and must be transported to the nucleolus.

Ribosomal proteins begin to associate with ribosomal RNAs before cleavage of 45S ribosomal RNA occurs. After separation, the remaining ribosomal proteins and 5S ribosomal RNA are added.

18S ribosomal RNA maturation occurs faster. Finally, the "preribosomal particles" are exported to the cytoplasm.

Other functions

In addition to the biogenesis of ribosomes, recent research has found that the nucleolus is a multifunctional entity.

The nucleolus is also involved in the processing and maturation of other types of RNA, such as snRNPs (protein and RNA complexes that combine with pre-messenger RNA to form the spliceosome or splice complex) and certain transfer RNAs., microRNAs, and other ribonucleoprotein complexes.

Through the analysis of the nucleolus proteome, proteins associated with pre-messenger RNA processing, cell cycle control, DNA replication and repair have been found. The protein constitution of the nucleolus is dynamic and changes under different environmental conditions and cell stress.

Likewise, there are a series of pathologies associated with the incorrect functioning of the nucleolus. These include Diamond – Blackfan anemia and neurodegenerative disorders such as Alzheimer's and Huntington's disease.

In Alzheimer's patients there is a change in the expression levels of the nucleolus, compared to healthy patients.

The nucleolus and cancer

More than 5000 studies have shown the relationship between malignant cell proliferation and nucleolus activity.

The goal of some investigations is to quantify nucleolus proteins for clinical diagnostic purposes. In other words, the aim is to evaluate cancer proliferation using these proteins as a marker, specifically B23, nucleolin, UBF and RNA polymerase I subunits.

On the other hand, it has been found that the B23 protein is directly related to the development of cancer. Likewise, other nucleolar components are involved in the development of pathologies such as acute promyelocytic leukemia.

The nucleolus and viruses

There is sufficient evidence to state that viruses, both from plants and animals, need nucleolus proteins to achieve the replication process. There are changes in the nucleolus, in terms of its morphology and its protein composition, when the cell experiences a viral infection.

A significant number of proteins have been found that come from DNA and RNA sequences that contain viruses and are located in the nucleolus.

Viruses have different strategies that allow them to locate in this subnuclear region, such as viral proteins that contain “signals” that lead them to the nucleolus. These tags are rich in the amino acids arginine and lysine.

The location of viruses in the nucleolus facilitates their replication and, furthermore, it seems to be a requirement for their pathogenicity.

References

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